Regeneration refers to the ability to replace tissue that has been lost with tissue that is functionally equivalent. Two fundamental questions need to be addressed with respect to the biology of regeneration: (1) how does regenerative growth differ from the growth that occurs during normal development? (2) Why can some tissues regenerate while others cannot? Drosophila imaginal discs, the larval primordial of adult structures such as the wing and the eye, are capable of regenerating after damage. Until recently, most studies of imaginal-disc regeneration involved culturing fragmented discs in the abdomens of adult female Drosophila. It was therefore difficult to conduct genetic screens for regulators of regenerative growth. Our laboratory has developed a way of studying regeneration in imaginal discs that does not require laborious surgical techniques. Expression of a pro-apoptotic gene for a short period in the developing wing pouch ablates most of the pouch tissue and leaves a rim of surviving cells. Regenerative growth restores the ablated pouch and adult flies can have wings of relatively normal size. An early event during regeneration is upregulation of the morphogen Wingless which results in increased levels of Myc protein; increasing Myc levels enhances regeneration. Additionally, regeneration occurs efficiently in discs from early third instar larvae but damage at the later third instar stage does not result in regeneration. Thus, as in many other organisms, a Wnt protein is upregulated during regeneration and the ability of imaginal discs to regenerate depends on the developmental stage. To understand how tissue damage initiates the process of regeneration, the properties of an enhancer of the wingless gene that is activated upon tissue damage will be characterized (Aim 1) and the signaling pathways that activate the enhancer will be defined. The mechanisms that prevent activation of the same enhancer in response to tissue damage in older larvae will also be characterized. Aim 2 seeks to identify and characterize genetic alterations that promote regeneration in older imaginal discs. Aim 3 will address the interrelationship between perturbations in imaginal disc growth and developmental timing.